1. Field of the Invention
The present invention relates to a technical field in respect of a light emitting device provided with an element having a structure interposing a thin film which emits light by a phenomenon called electroluminescence (hereinafter, referred to as “EL”) between an anode and a cathode over a substrate.
2. Description of the Related Art
An EL element is an element which emits light by forming a thin film or a crystal containing an organic compound or an inorganic compound between a cathode and an anode and by flowing current between the cathode and anode. In recent years, an EL element in which a thin film (hereinafter, referred to as an organic thin film) chiefly containing an organic compound as a main constituent is located between a cathode and an anode, namely an organic EL element, has been actively developed.
The organic EL element is expected to be applied to various fields and is considered that the use thereof ranges from only a lightning appliance to a display used for a cellular phone and a personal computer, or the like. The organic EL element has been partially put to practical use as a display for automobile use, the display portion of a small electric appliance, and the like.
However, some problems on the organic EL element remain regardless of how the organic EL element is used. One of the problems is display failure due to a manufacturing process. Specifically, as shown in FIG. 1A, the organic EL element is a device in which a thin film containing an organic compound is located between two electrodes. In the organic EL element, electrons are injected from one electrode and holes are injected from another electrode to drive the organic EL element. Then, a hole and an electron are recombined in the thin film to form the excited state of a compound for light emission, namely a luminous body. Light emission can be obtained when the excited state returns to the ground state in a radiation step. That is, the organic EL element is a light emitting element which is driven only by the flow of current. Therefore, when current does not flow to a pixel or when current can not flow uniformly to a pixel by short circuit between two electrodes, light emission as an element is not obtained.
In particular, the short circuit between electrodes is a major concern. Several causes of the short circuit are considered. Crystallization of a material in a film and concentration of an electric field derived from the unevenness of the electrode surface, namely the surface roughness of the electrode surface can be given as main causes. The former cause can be solved by selecting a material which is difficult to be crystallized; however, the latter cause can not be solved only by contriving the material since the cause is attributable to a manufacturing process. For example, in the case where minute dust is generated during the process of manufacturing a substrate and then a first electrode is formed with the dust attached (FIG. 1B), a projection is formed in this electrode. When an EL element is formed over a pixel electrode having such a rough surface, an electric field concentrates in this projection and current required for light emission can not flow to other regions, and consequently, spots are generated in the light emission of the pixel. In addition, when the concentration of an electric field further proceeds, the element is destroyed and short-circuited between two electrodes. As a result, this pixel can not emit light in most of the region.
For example, in the case of applying the organic EL element as a high precision display, an EL element is manufactured in each pixel over the substrate of a simple matrix type (hereinafter, referred to as a passive matrix) or an active matrix type. Display can be performed by controlling current supplied to each pixel using an external circuit. At this time, when the pixel is short-circuited by the above problem due to the process, the pixel is recognized as a black spot (hereinafter, referred to as a blind spot) on the screen, and the display quality is diminished.
In the case of considering the application of the organic EL element for lightning, dust generated during the process can be removed with comparatively ease since the process for manufacturing a substrate is comparatively simple. However, in the case of considering the application for a display, minute dust generated during the process is hard to be completely removed since the process for manufacturing an active matrix type substrate is extremely complicated in particular. Especially when the panel is grown in size, subtle dust greatly influences on a yield; therefore, there is an urgent need to develop a method for forming an EL element which is not influenced by the surface roughness of a pixel electrode due to dust.
In a conventional pixel structure shown in FIG. 2A, reference numeral 201 denotes a data signal line; 202, a gate signal line; 203, a power source line; 204, a switching TFT; 205, a capacitor; 206, a driving TFT; 207, a drain electrode of the driving TFT; and 208, a pixel electrode connected to the drain electrode of the driving TFT. The pixel electrode 208 serves as an anode of a light emitting element.
FIG. 2B shows a figure corresponding to a cross section in FIG. 2A taken along line A-A′. An insulator 209 patterned in a lattice shape is provided, so as to cover the edge portion of the pixel electrode 208 formed to be connected to the driving TFT 206 and at least the driving TFT and the switching TFT.
In FIGS. 2A and 2B, dust does not exist below the pixel electrode, and the pixel electrode 208 having high planarity is provided. Therefore, light emitting layers 211a to 211c formed over the pixel electrode 208 are not required to be thickly deposited on purpose. The thickness of the light emitting layer at this time is generally from 10 nm to 40 nm and approximately 100 nm at most. A cathode 212 is formed thereover.
FIG. 3A shows a schematic view in the case where the planarity of a pixel electrode is lost since dust due to the process exists below the pixel electrode. In FIG. 3A, reference numeral 300 denotes a substrate, and reference numerals 301a to 301c denote pixel electrodes. The figure schematically shows that dust 302 due to the process exists below the pixel electrode 301a. The planarity of the pixel electrode 301a formed in this pixel portion is lost and a large projection shape is formed since the dust 302 is not completely removed.
The film thickness of a general EL element is generally from 0.1 μm to 0.2 μm and approximately 0.5 μm at most. Therefore, in the case of manufacturing an EL element over an anode having a projection by a usual method, as shown in FIG. 3B, light emitting layers 303b and 303c having high planarity are formed over the pixel electrodes 301b and 301c; however, the planarity of the light emitting layer 303a formed over the pixel electrode 301a is lost and a large projection is formed. Further, a cathode 304 is formed over the light emitting layer formed as above; however, when current is flown in each pixel, light emission can not be virtually obtained in a region in which the pixel electrode 301a exists although light emission can be obtained in a light emitting region in which the pixel electrodes 301b and 301c exist. This is because the concentration of an electric field is easy to occur in a projection in the region the pixel electrode 301a exists, and the reliability of such a pixel is severely decreased. That is, this pixel region is recognized as a blind spot.